Others seem to have interpreted this as "find all groups of one or more M's".

On the off chance that you actually meant 6 or more M's, try this modification of japhy's solution:

my $seq = "...";
my @groups;
push @groups, [$-[0], $+[0]-1] while $seq =~ /M{6,}/g;
print "$_->[0] to $_->[1]\n" for @groups;
[download]

(where the displayed positions are 0-based.)